Air by-pass pressure sifter

ABSTRACT

A method and apparatus for increasing the sifting capacity of a pressurized oscillating sifter by directing upwardly a portion of the pressurized air conveying particulate material through a bypass screen of the same mesh as the sifting screen and into a bypass channel. The by-pass air is then utilized to entrain the undersize materials which passed through the screen (also called &#39;&#39;&#39;&#39;under,&#39;&#39;&#39;&#39; &#39;&#39;&#39;&#39;thrus&#39;&#39;&#39;&#39; and &#39;&#39;&#39;&#39;fines&#39;&#39;&#39;&#39;) and convey it to further processing stations. Bed depth on the top screen may be increased by providing a weir at the outlet for oversize material (also called &#39;&#39;&#39;&#39;overs&#39;&#39;&#39;&#39; and &#39;&#39;&#39;&#39;tailings&#39;&#39;&#39;&#39;). Carryover of high-velocity fines into the overs is prevented by providing an impact barrier at the overs outlet on the last screen.

United States Patent Krolopp et a1.

[ AIR BY-PASS PRESSURE SIFIER Filed:

Inventors: Otto C. Krolopp; John W. Angstadt,

both of Williamsville, NY.

Assignee: Blaw-Knox Food and Chemical Equipment, Inc., Buffalo, NY.

May 6, 1971 Appl. No.: 140,718

FOREIGN PATENTS OR APPLICATIONS 839,152 4/1952 Germany 209/316 854,597l/l940 France 905,804 9/1962 Great Britain 209/28 PrimaryExaminer-Robert Halper Attorney, Agent, or FirmMarn & .langarathis [5 7]ABSTRACT A method and apparatus for increasing the sifting capacity of apressurized oscillating sifter by directing upwardly a portion of thepressurized air conveying particulate material through a by-pass screenof the same mesh as the sifting screen and into a by-pass channel. Theby-pass air is then utilized to entrain the undersize materials whichpassed through the screen (also called under, thrus and fines) andconvey it to further processing stations. Bed depth on the top screenmay be increased by providing a weir at the outlet for oversize material(also called overs" and tailings). Carryover of high-velocity fines intothe overs is prevented by providing an impact barrier at the oversoutlet on the last screen.

12 Claims, 5 Drawing Figures PATENYED MR 1 2 I974 SHEET 2 BF 3 OversUnders By-Puss Air 64 Fig. 2.

INVENTORS 0H0 C. Krolopp John W. Angstudt Y 77Za/m&

ATTORNEYS PATENIEDHAR 12 um 3796312 SHEET 3 0F 3 Fig. 5.

INVENTORS 0H0 C. Krolopp John W. Angstodt ATTORNEYS AIR BY-PASS PRESSURESIFTER BACKGROUND OF THE INVENTION This invention relates generally toclassification apparatus and, more particularly, to machines referred toas rotary pressure screens or rotary pressure sifters. Such machines arewidely employed for the sizeseparation of dry particulate material, andfind particular applications in the food, feed and chemical processindustries.

Broadly, rotary pressure sifters comprise a pressure vessel divided intoupper and lower chambers separated by a'screen or screens of desiredmesh. The vessel is mounted in such a manner that it can be horizontallyoscillated to cause the particles to move across the screen to effectthereby the sifting action. The function of this movement is totransport the oversize particles across the screen to an outlet fortheseparticles. It will be appreciated that the invention is applicableto essentially any type of pressure classification device, withoutregard to the type of screen motion employed. An inlet in the upperchamber is connected by a flexible coupling to the source of thepneumatically-conveyed particulate material. Generally, a baffle of somesort is provided near the inlet, as regulations governing the sifting ofcertain foods, for instance, require a true sifting action and prohibituse of the air velocity to literally blow particles through the screen.All of the air passes either through the screen or into the oversdischarge outlet. Outlets are also provided with flexible couplings soas to be operable during oscillating movement. Frequently, a pluralityof screens are provided, withthe overs from the first screen passingdownwardly to a second, and so forth. Each screen increases theefficiency of the separation. Alternatively, multiple screens ofdiffering mesh can be employed to produce multiple cuts orclassifications, resulting in the production of overs, middles andunders. A typical prior art pressure sifter is disclosed in US. Pat. No.3,250,389 of Scruby et al. Generally, all screens slope slightly towardthe overs outlet, so that gravity facilitates movement thereover, butthis is optional, and flat screens are also used.

Since air must pass through the screen of a conventional pressuresifter, it is necessary that open areas be maintained on the screen atall times. This of course places a limit on the rate at whichparticulate material can be fed to the machine, i.e., it limits thethroughput.

OBJECTS OF THE INVENTION It is a general object of the present inventionto provide an improved pressure sifter and sifting method.

Another object of the present invention is to provide a pressure sifterof significantly greater sifting capacity than prior art sifters of thesame general size and type.

A further object of the present invention is to provide means forsifting a deep bed of particulate material, thereby increasing siftingefficiency.

Still another object of the present invention is to provide means forseparating air from the particulate material during sifting, wherebysifter capacityis increased.

Various other objects and advantages of the invention will become clearfrom the following description of embodiments thereof, and the novelfeatures will be particularly pointed out in connection with theappended claims.

BRIEF DESCRIPTION OF DRAWINGS Reference will hereinafter be made to theaccompanying drawings illustrating a preferred embodiment of theinvention, and wherein:

FIG. 1 is a simplified schematic diagram of a sifter employing theinvention;

FIG.2 is an exploded perspective view of the operative parts of thesifter, showing the flow of overs, unders and air;

FIG. 3 is a schematic representation of air flow in the vicinity of theinlet;

FIG. 4 is a schematic representation of overs flow across the bottomscreen, illustrating the effect of the impact barrier; and

FIG. 5 is a cross-sectional elevation of a preferred embodiment of theinvention.

DESCRIPTION OF PREFERRED EMBODIMENT A rotary pressure sifter having top,middle and bottom screens is described. Broadly, the objects of theinvention are carried out by providing a by-pass screen of the same meshas the top screen, located above the top screen. Air entering the inletchamber of the sifter can pass upwardly through this by-pass screen andmay carry some air-borne particulate unders. The chamber above theby-pass screen communicates with a vertical passage into which theunders from all the screens are passed, which in turn communicates withthe unders outlet in the bottom portion of the machine. A selfcleaningweir is optionally provided around the overs discharge opening in thetop screen, thus increasing bed depth in this area. An air impactbarrier is provided on one side of the overs discharge outlet on thebottom screen, to prevent discharge of any high velocity particlestherethrough without thorough screening.

With reference to FIGS. 1 and 2, the rotary pressure sifter comprises adomed cover 10, a dished bottom 12 and a cylindrical center section 14therebetween. These three sections are joined by well-known means toform a pressure tight vessel (it is to be noted that pressures employedare low, about 5-15 psig, so heavy vconstruction is not required).

Cover 10 has a centrally located inlet pipe 16 adapted for connection onthe outside-with a suitable flexible coupling (not shown). An angledbaffle 18 located below inlet 16 deflects the incoming particlebearingair stream in a manner described below in connection with FIG. 3. Avertical wall 20 attached to the underside of cover 10 and a crossmember 22 spanning a chord of cover 10 support the horizontal by-passscreen 24 and define the by-pass chamber 26. These are the essentialelements of cover 10.

A top screen 28 is located in the top of central section 14 and iscomprised of a supporting ring 30, screen 32, a by-pass air passage 34and an overs discharge opening 36. A weir 38 may be provided aroundovers discharge opening 36, if desired, to increase bed depth. Thereshould be some clearance under weir 38, however, to allow screen 32 tocompletely self-clean on shutdown. The lower edge of vertical member 22in cover 10 adjoins the edge of air passage 34 to form a closed passagefor by-pass air around screen 32.

A top tray 40 is located directly below top screen 28, and is comprisedof a support ring 52 and tray member 44%.. The latter element includes awalled opening 46 directly beneath overs discharge opening 36 in tray28,

so as to form a closed vertical passage for'overs through tray 40. Traymember as is suitably sloped so as to catch unders passing throughscreen member 32 and direct them by gravity flow to unders dischargeopening 48 located directly below by-pass air passage 34 in top screen28. Discharge opening 48 is only half the size of air passage 34, forreason which will become apparent hereinbelow.

A middle screen 50 is located directly below top tray 40. Middle screen50 is comprised of a support ring 52, a screen member 54 and a pair ofopenings 56, 58. Opening 56 is located directly below unders dischargeopening 48 in tray member 44, and'because of the slope in tray member 44opening 48 is in the bottom of top tray 40, so opening 56 forms a closedpassage for unders past middle screen 5.0. A vertical member 60separates openings 56 and 58, which are adjacent each other. Opening 58forms the overs discharge passage for middle screen 50.

A middle tray 62 is located directly below middle screen 50 and iscomprised of a support ring 64, the sloped tray member 66, undersdischarge opening 68 and walled overs passage 70. Tray member 66underlies all of screen member 54, and is sloped so as to guide understo unders discharge opening 68 which is directly below opening 56 inmiddle screen 50. Walled overs passage 70 is directly below opening 58in middle screen 50 and defines a closed passage for overs down pastmiddle tray 62.

A lower screen 72 is located directly below middle tray 62 and iscomprised of'a support ring 74, a screen member 76, overs dischargeopening 78, an impact barrier 80 and an unders passage 82. Underspassage 82 cooperates with unders discharge opening 68 in middle tray 62in the same manner as openings 48 and 56 cooperate, as described above,to fonn a closed passage for unders past lower screen 72. Oversdischarge opening 78 is diametrically opposite to unders passage 82 andis partially screened by impact barrier 80, which is ,a vertical wallextending to the underside of middle tray member 66. The function ofimpact barrier 80 is described hereinbelow in connection with FIG. 4.

The bottom section 12 of the machine (FIG. ll) has an internal divider84 separating overs passage 86 from unders passage 88. Overs passage 86is directly below the overs discharge opening 78 in bottom screen 72,and communicates with overs outlet 90. Unders passage 88 underlies lowerscreen 72 and is in sloping communication with unders outlet 92.

In FIG. 2, a heavy line has been used to trace the course of oversthrough the sifter, a light line traces the course of unders, and adotted line traces the by-pass air. Thus, overs move across top screen32 and drop through passages 36 and 46 and onto middle screen 54; acrossscreen 54 and drop through passages 58 and 70 and onto lower screen 76;across screen 76, around impact barrier 80, downv through passage 78 andout of the machine. Passage of overs across screens 32, 54 and 76 isaccomplished by gyratory motion and the slight sloping toward theoutlets. The unders pass through each of the screens 32, S4 and 76 ontothe respective trays 66 and bottom 88, and into the bypass air passage,where they are entrained by lay-pass air. The vertical by-pass airpassage is formed by openings 34, 48, 56, 68 and 82.

The effect of angled deflector H8 is graphically illustrated in H6. 3.As indicated by the arrows, the pneumatically conveyed product streamenters inlet 16 and is deflected toward the periphery in a directionparallel to the long side of overs discharge outlet 36 and also parallelto the long side of air by-pass screen 24. In practice, the velocity ofthe air is reduced to about l/64 of the input velocity in this manner.This sudden reduction of velocity allows the product in suspension toslow down rapidly and fall to the surface of top screen 32 below. Impactof particles on the underside of cover 10 also slows particles down andcauses them to drop to the screen. These measures minimize the number ofunders particles falling directly through overs discharge opening 36(but of course do not eliminate this). Since the top screen 32 iscovered entirely with particles, the path of least resistance for theair is through by-pass screen 24 (the normal gyratory motion of thescreen levels particles-on all of the screens into an even covering).

It is possible that overs particles moving onto bottom screen 72 mayhave enough residual velocity to move directly across screen member 76and into final overs outlet 78. To prevent this, impact barrier 80 ispro-.

(not shown), all of which are entirely conventional and need not bedescribed herein.

In actual tests, a 48 inch rotary pressure sifter equipped with the airby-pass features of the present invention and sifting spring patentwheat flour through 2160 mesh stainless steel B.C. mesh wire, was foundto have a capacity of 40,000 pounds per hour. Priorto installation ofthe air by-pass and other features of the invention, the same unit had acapacity of only 13,000

pounds per hour under similar conditions.

Various changes in the details, steps, materials and arrangements ofparts, which have herein been described and illustrated in ordertoexplain the nature of the invention,may be made by those skilled in theart within the principle and scope of the invention as defined in theappended claims. In particular, it will be appreciated that theinvention is applicable to single screen sifters, multiple cut sifters,and the like, as well as the embodiment described herein.

What is claimed is:

l. A pressure sifter for size classification of particulate materialconveyed by a pneumatic medium comprising:

I a pressure tight chamber having inlet means in the upper portionthereof and at least one outlet means in the lower portion thereof;

drive means for imparting oscillatory movement to said chamber, in ahorizontal plane;

a sifting screen laterally disposed within said chamber and dividing theupper portion from the lower portion;

a by-pass conduit means, the inlet to which conduit means being disposedabove said sifting screen and in gaseous communication with the lowerportion of said chamber beneath said sifting screen and with said outletmeans for passing a portion of said pneumatic medium therethrough; and

an oversize particle conduit means in particulate communication with theupper surface of said sifting screen and another outlet means.

2. The sifter as defined in claim 1 wherein a screen is disposed in saidupper portion of said chamber between said inlet thereto and the inletto said by-pass conduit means.

3. The sifter as claimed in claim 2 wherein said bypass conduit meansand screen therefor and said sifting screen are of the same mesh.

4. The sifter as claimed in claim 1 and additionally comprising bafflemeans adjacent said inlet means adapted to deflect particulate materialentering said inlet means.

5. The sifter as claimed in claim 1 wherein said sifting screen issloped downwardly toward said oversize particle conduit.

6. The sifter as claimed in claim 1 and additionally comprising traymeans disposed below said sifting screen and sloped downwardly towardsaid by-pass conduit means.

7. The sifter as claimed in claim 6 and additionally comprising at leastone additional sifting screen in said lower portion and below said traymeans, the upper surface of each of said additional sifting screensbeing in particulate communication with said oversize particle conduit,the upper surface of each said additional screen including an openingfor passage of oversize particles to a lower screen or the other of saidoutlet means, and the lower side of each of said additional screensbeing in fluid communication with said by-pass conduit means.

8. The sifter as claimed in claim 7 wherein the opening in the lowest ofsaid additional screens for passage of oversize particles is partiallywalled off by a vertical impact barrier.

9. The sifter as claimed in claim 1 wherein said oversize particleconduit means terminates in an opening in the upper surface of saidsifting screen and additionally comprises a weir at least partiallysurrounding said opening.

10. The sifter as claimed in claim 1 wherein said bypass conduit meansis in fluid communication with said outlet means for undersize particlesand said oversize particulate conduit is in fluid communication with theother of said outlet means.

11. In the operation of a pressure sifter driven to impart oscillatorymovement in a horizontal plane and formed of a pressure tight chamberincluding a sifting screen wherein a pneumatically conveyed particulatematerial is introduced into the chamber and dropped onto said screen andwherein a portion of the particulate material is passed through saidscreen, the improvement comprising: reducing the velocity of theparticulate material before dropping it on said screen; withdrawing amajor portion of the pneumatic conveying medium from a zone above saidsifting screen and passing said major portion through a by-pass conduitto a zone beneath said screen wherein said major portion is utilized topneumatically re-entrain and withdraw from said sifter said portion ofsaid particulate material.

12. The process as defined in claim 11 wherein said major portion ofpneumatic conveying medium is passed through a by-pass screen of thesame mesh size as said sifting screen and positioned above said siftingscreen prior to passage through said by-pass conduit.

1. A pressure sifter for size classification of particulate materialconveyed by a pneumatic medium comprising: a pressure tight chamberhaving inlet means in the upper portion thereof and at least one outletmeans in the lower portion thereof; drive means for impartingoscillatory movement to said chamber, in a horizontal plane; a siftingscreen laterally disposed within said chamber and dividing the upperportion from the lower portion; a by-pass conduit means, the inlet towhich conduit means being disposed above said sifting screen and ingaseous communication with the lower portion of said chamber beneathsaid sifting screen and with said outlet means for passing a portion ofsaid pneumatic medium therethrough; and an oversize particle conduitmeans in particulate communication with the upper surface of saidsifting screen and another outlet means.
 2. The sifter as defined inclaim 1 wherein a screen is disposed in said upper portion of saidchamber between said inlet thereto and the inlet to said by-pass conduitmeans.
 3. The sifter as claimed in claim 2 wherein said by-pass conduitmeans and screen therefor and said sifting screen are of the same mesh.4. The sifter as claimed in claim 1 and additionally comprising bafflemeans adjacent said inlet means adapted to deflect particulate materialentering said inlet means.
 5. The sifter as claimed in claim 1 whereinsaid sifting screen is sloped downwardly toward said oversize particleconduit.
 6. The sifter as claimed in claim 1 and additionally comprisingtray means disposed below said sifting screen and sloped downwardlytoward said by-pass conduit means.
 7. The sifter as claimed in claim 6and additionally comprising At least one additional sifting screen insaid lower portion and below said tray means, the upper surface of eachof said additional sifting screens being in particulate communicationwith said oversize particle conduit, the upper surface of each saidadditional screen including an opening for passage of oversize particlesto a lower screen or the other of said outlet means, and the lower sideof each of said additional screens being in fluid communication withsaid by-pass conduit means.
 8. The sifter as claimed in claim 7 whereinthe opening in the lowest of said additional screens for passage ofoversize particles is partially walled off by a vertical impact barrier.9. The sifter as claimed in claim 1 wherein said oversize particleconduit means terminates in an opening in the upper surface of saidsifting screen and additionally comprises a weir at least partiallysurrounding said opening.
 10. The sifter as claimed in claim 1 whereinsaid by-pass conduit means is in fluid communication with said outletmeans for undersize particles and said oversize particulate conduit isin fluid communication with the other of said outlet means.
 11. In theoperation of a pressure sifter driven to impart oscillatory movement ina horizontal plane and formed of a pressure tight chamber including asifting screen wherein a pneumatically conveyed particulate material isintroduced into the chamber and dropped onto said screen and wherein aportion of the particulate material is passed through said screen, theimprovement comprising: reducing the velocity of the particulatematerial before dropping it on said screen; withdrawing a major portionof the pneumatic conveying medium from a zone above said sifting screenand passing said major portion through a by-pass conduit to a zonebeneath said screen wherein said major portion is utilized topneumatically re-entrain and withdraw from said sifter said portion ofsaid particulate material.
 12. The process as defined in claim 11wherein said major portion of pneumatic conveying medium is passedthrough a by-pass screen of the same mesh size as said sifting screenand positioned above said sifting screen prior to passage through saidby-pass conduit.